Install Guide: Edits on Networking chapter

Adds procedure, improves text.
Changes external link to Ubuntu Install Guide to internal link.

this is taken from https://review.openstack.org/#/c/52976/2 and
taken as separate patch

backport: havana
Cherry-Picked: https://review.openstack.org/#/c/53051/
Closes-Bug: #1243132
Change-Id: If24cf613b57bbc1f5d214f3522b1b0a158703ded
Merge "Install Guide: Edits on Networking chapter" into stable/havana